“The 2014 campaigns are currently dominated by interest group advertising,” Erika Franklin Fowler, co-director of the Wesleyan Media Project, said. “And the bulk of the interest group money is coming from dark money groups who don’t disclose their donors.”

The research shows more than $43 million has already been spent on nearly 110,000 television ads focused on Senate campaigns. The total number of ads represents a 45% increase over commercial airings at this point in the cycle in 2012.

Republicans have benefitted more from the dark money ads than Democrats. About 67% of ads by outside groups favored GOP Senate candidates, while only 49% supported Democratic Senate candidates.